The present invention relates generally to stringed musical instruments, reluctance pickups for stringed musical instruments and instrument equipment. More particularly, this invention pertains to guitars, guitar pickups, and guitar equipment. Even more particularly, this invention pertains to digital guitars, multi-signal guitar pickups, and digital guitar interface devices.
String instruments, such as guitars, are well known in the art and include a wide variety of different types and designs. For example, the prior art includes various types of acoustic and electric guitars. These guitars are typically adapted to receive analog audio signals, such as analog microphone signals, and to output analog audio signals, such as analog string signals (analog audio signals generated by guitar pickups when guitar strings are strummed).
The prior art includes monophonic guitars, i.e., guitars that output a single string signal when one or more of the guitar strings mounted on the guitar are strummed. The prior art also includes guitars that output a single string signal for each string mounted on a guitar. The latter type of guitar is generally referred to as a polyphonic guitar.
The traditional guitar has a plurality of guitar strings that are secured at each end and held under tension to vibrate at the appropriate frequency. The guitar strings are supported on a bridge over a transducer or pickup. In a polyphonic pickup, each sensor is dedicated to a different string of the guitar. The two common types of pickups used for this purpose are piezoelectric and magnetic pickups. On electric guitars with magnetic polyphonic pickups, the guitar strings normally do not touch the pickups. Each transducer typically includes a permanent magnet that creates a magnetic field and an electrical coil that is placed within the magnetic field. For each transducer, the corresponding strings are constructed from magnetically permeable material and the transducer is mounted upon the guitar so that at least one selected string passes through each transducer's magnetic field. When the instrument is played, the string vibrates causing the magnetically permeable material to move through the magnetic field so as to produce an oscillating magnetic flux at the windings of the corresponding coils. Thus, through magnetic induction, the vibration of the guitar strings moving within the lines of magnetic flux emanating from the pickup causes an electrical signal to be generated with the coil of the pickup.
Variable reluctance type transducers are often used to measure or detect the velocity of a moving ferromagnetic target. When the target has only one degree of freedom, such as movement in an up or down direction, the direction of velocity of the target can be determined from the polarity of the voltage induced at the sensing coil of the transducer and the magnitude of the velocity is proportional to the sensed voltage. However, if the target, such as a selected length of a vibrating guitar string, has two degrees of freedom, then the target can move in either an up or down direction or a left to right direction or any vector combination thereof. Such movement of the string at any one point along its length is described as a variable vector in the X-Y plane normal to the string at that point. This variable vector is separable into an x-component vector and a y-component vector, where the x and y axis are arbitrary Cartesian axial directions. Using a single conventional reluctance transducer with a symmetric magnetic field, the direction of movement cannot be determined from the induced voltage polarity, nor does the magnitude of the induced voltage accurately represent the magnitude of the target's velocity.
When a guitar string is plucked and released, a given point on the string vibrates in multiple directions in the transverse plane. The transverse plane, or X-Y plane, is the plane perpendicular to the axis of the string. The path of string vibration may be, for example, a precessing ellipse in the X-Y plane. Conventional magnetic polyphonic guitar pickups respond primarily to string vibrations occurring along a primary axis, such as the vertical axis—towards and away from the pickup. They also respond, but with less sensitivity, to string vibrations occurring along a secondary axis normal to the primary axis, such as the horizontal or axis—in the plane defined by the strings. As a result of this cross-axis insensitivity, string vibrations in different directions induce differently scaled voltages in the sensing coil that are inseparably mixed in the output signal. This drawback of conventional, single transducer magnetic pickups limits the measurable performance parameters of the pickups, including: frequency response, and dynamic response (i.e. signal-to-noise ratio response). As a demonstrative example, string vibrations with large amplitude in a near-horizontal direction may be indistinguishable from those with small amplitude in a near-vertical direction. The pickup may respond with different sensitivities to string vibrations of equal amplitudes in different directions.
The insufficiency of conventional guitar pickups in representatively sensing transverse string vibration in two degrees of freedom has been recognized by other inventors in the prior art. An example of a multiple pole pickup for a single string is shown in U.S. Pat. No. 4,348,930 issued to Chobanian et al. on Sep. 14, 1982 entitled Transducer For Sensing String Vibrational Movement in Two Mutually Perpendicular Planes. This patent teaches separate dedicated pole pieces and coils that are sensitive to vibration in two separate and mutually perpendicular planes. It is claimed that when the string vibrates in the sensitive plane of one of the sensors, significantly greater changes result in the magnetic flux in one pole piece than in the other pole piece.
With U.S. Pat. No. 4,534,258, entitled Transducer Assembly Responsive to String Movement in Intersecting Planes, Norman J. Anderson describes a magnetic pickup designed to determine all the transverse movement of the string. In this design, too, each coil is maximally sensitive to vibration of the string in a first plane and minimally sensitive to vibration of the string in a second plane that intersects the first plane. Anderson explains that these principal planes are preferably perpendicular and at −45 degree and +45 degree angles with respect to the top surface of the guitar body. The signals induced by the vibrations of all strings in one set of coils are combined into one audio channel, and signals induced by the vibration of all strings in the other set of coils are combined into the second audio channel.
U.S. Pat. No. 5,206,449 entitled Omniplanar Pickup for Musical Instruments, Richard E. D. McClish describes a similar arrangement of magnetic sensors, to achieve omniplanar sensitivity to string vibration. According to that invention the signals from two coils are combined after a phase shift is applied to one of the signals with respect to the other. The flux fields are coupled by proximity and they intersect at the string, go that both sensor coils respond to string vibration in any direction, and they respond with different levels of sensitivity.
U.S. Pat. No. 6,392,137 to Isvan, and assigned to the assignee of the present invention, describes a three coil pickup which is sensitive to both the vibrations in the string plane and the vibrations perpendicular to the string plane. The Isvan pickup includes two pickup coils, each with a pole piece of like polarity and biased horizontally in opposite directions from each other, and a third pole piece having an opposite polarity. The Isvan electronic system subtracts the signals from the first and second coils to create a signal representing the vibrations in the string plane and combines the signals from the first pickup and the second pickup for determining the string vibrations perpendicular to the string plane. In one embodiment of the invention, the transducer uses one pole of the pickup as a bridge saddle for supporting the guitar string. The saddle pole of the pickup is constructed from a magnetically permeable material. The saddle pole causes the lines of magnetic flux to be carried in large part by the guitar string and allows for a reduction in the total magnetic energy requirement for the pickup's permanent magnet to reduce the cross talk between adjacent string sensors within a polyphonic pickup.
Each of the prior art patents cited above attempt to solve the X-Y sensing problem, with varying degrees of success, by resolving the variable vector of string vibration onto orthogonal axes sensed differently by the two or more coils of a pickup. Depending on the prior art system, the x-motion and y-motion components are either directly measured as separate coil signals each proportionate to either an x-motion vector or a y-motion vector or, the x-motion and y-motion components are electronically separated by phase shifting or other signal processing of the coil signals. Both prior art approaches have drawbacks. One approach requires more complicated coil configurations, the other approach requires more complicated electrical processing.
What is needed, then, is a transducer for a vibratory string that is particularly directed towards a simple, cost-effective means of optimizing X-Y motion sensing, and thus the transducer's measurable performance parameters, including: frequency response, dynamic response (i.e. signal-to-noise ratio response).
These prior art magnetic polyphonic pickups may also suffer from significant magnetic cross talk between the strings because of coil arrangement and sensitivity. Cross talk can occur when a transducer senses the vibration of adjacent strings in addition to the one immediately overlying the transducer in question. This may be caused by the second string's vibrations affecting the magnetic field at the coils of the first transducer, and may also be caused by stray magnetic flux of the second transducer affecting the readings of the first transducer's coils.
What is needed, then, is a transducer for a vibratory string that is particularly directed to providing a simple, cost-effective means of reducing cross talk between strings while optimizing X-Y motion sensing, and thus the transducer's measurable performance parameters, including: frequency response, dynamic response (i.e. signal-to-noise ratio response).